Technology update

Jun 7, 2007

Nanobubbles go on and on

Surface nanobubbles are extremely stable, contrary to what was previously believed, say physicists in the Netherlands. Detlef Lohse of the University of Twente and colleagues have found that these bubbles are not only stable at room temperature and pressure but also under extreme negative pressures down to –6 MPa. This "super-stability" could be important for understanding liquid–solid interfaces and may even find applications in industry.

Researchers recently found evidence for the existence of stable nanobubbles located at the interface between a solid and liquid. These surface nanobubbles, which are between 10 and 100 nm across, are puzzling objects because they should not exist. According to experimental data, the bubbles have a radius of curvature of 100 nm and should therefore dissolve in less than a second because of a large Laplace pressure inside the bubbles.

Now, Lohse and co-workers have found that these bubbles remain stable even when a large negative pressure of –6 MPa is applied to them. Contrary to what was previously believed, the bubbles do not act as nucleation sites for cavitation on surfaces – which would cause them to collapse because of large tensile stresses created in the surrounding liquid.

The Twente team made its nanobubbles by submerging hydrophobic silicon substrates in water so that bubbles of air "popped" out of solution at the solid–liquid interface. Next, the researchers applied a shock wave to the silicon substrates, which generated a large tensile stress of around –6 MPa in the water. They then monitored the cavitation activity with high-speed and AFM imaging.

To their surprise, the scientists found that the surface nanobubbles did not act as nucleation sites for cavitation bubbles. Instead, cavitation came from contamination or from microscopic structures, such as microcracks or microcrevices, on the silicon. According to the team, this means that the bubbles are unexpectedly stable under large tensile stresses.

"The thinking that surface nanobubbles have anything to do with cavitation has to be modified," Lohse told nanotechweb.org. "Everyone (including us) had expected that surface cavitation of the bubbles was related to their density, but we have shown that this is not the case."

The result will be important for explaining various phenomena associated with the solid–liquid interface, such as liquid slippage at walls or the anomalous attraction of hydrophobic surfaces in water, say the researchers. The super-stable nanobubbles might also help stabilize emulsions used in industry.

The team now plans to study different surfaces with varying contact angles. "We are also performing a better statistical analysis of our data," said Lohse.